This investigation will examine the Fc-receptor function of isolated leukocyte subpopulations (monocytes, lymphocytes, and granulocytes) from patients with systemic lupus erythematosus (SLL) and rheumatoid arthritis (RA). The characterization of Fc-receptor function will include: a) determination of membrane binding to antibody sensitized target cells, (EA rosette formation), and b) determination of post target cell binding cytolytic capability against antibody sensitized erythrocyte and tumor target cells (ADCC). Each leukocyte subpopulation will be examined in regards to subtle functional Fc-receptor defects by a) determining the minimum number of antibody molecules/target cells required to allow Fc-receptor binding and lysis (radioimmunoassay), and b) determining the affinity of leukocyte Fc-receptors for their ligand on sensitized target cells (competitive inhibition of ADCC). The reversibility of leukocyte Fc-receptor dysfunction both in vitro and in vivo will be determined as well as the isolation and characterization of factors in patient serum which may influence Fc-receptor function. Finally, effector cell Fc-receptors will be quantitated and characterized using hybridoma derived monoclonal antibody to isolated Fc-receptors.